devitrification: one construct, many systems

In geology devitrification is the slow crystallization of natural volcanic glass: obsidian and other rapidly quenched lavas are metastable, and over time, or with mild reheating and water, they convert to fine aggregates of crystalline silica and feldspar, producing characteristic spherulitic and felsitic textures. The glass does not melt and refreeze; it transforms in the solid state from an amorphous to a crystalline arrangement. Marshall's 1961 Devitrification of Natural Glass catalogued these textures and the conditions that produce them in volcanic rocks. The referent is the in-place crystallization of a geological glass, read from the textures it leaves behind, the same transformation that spoils a glassmaker's melt.[1]

Unit

The extent and texture of crystallization developed in a natural glass over time

Recovery means

Not applicable

Measured by

Read from spherulitic and felsitic textures, crystallite size, and the degree of conversion from glass to crystal

In glass science and manufacture, devitrification is the same transformation treated as a defect: because a glass is a frozen, metastable liquid, holding it in the temperature range where nucleation and crystal growth are both fast lets crystals form in what should remain a uniform amorphous solid, clouding the glass and weakening it. Glassmakers avoid it by moving the melt quickly through that danger range and by choosing compositions that resist crystallization. Shelby's Introduction to Glass Science and Technology frames devitrification through nucleation-and-growth kinetics and the critical cooling rate needed to keep a glass glassy. The referent is the onset of crystallinity in a manufactured glass, the identical physics as in a cooling lava, here fought rather than studied for its textures.[2]

Unit

The fraction of crystalline phase forming in a glass held in the crystallization range

Recovery means

Not applicable

Measured by

Tracked by nucleation and crystal-growth rates, time-temperature-transformation behaviour, and the critical cooling rate